The most identified strain of the virus is influenza A (H3N2). According to a CDC emergency health advisory released Dec. 27, 83 percent of reported cases were N3N2, a strain associated with more hospitalizations and deaths in those over 65 years of age and in young children compared to other age groups.

The agency notes that this year's vaccine is only expected to be about 32 percent effective because H3N2 tends to mutate. The strains used in the vaccine are determined months before the season actually begins so it's difficult to be 100 percent accurate.

"How well the vaccine works can depend in part on the match between the vaccine virus used to produce the vaccine and the circulating viruses that season," the CDC notes. "It’s not possible to predict what viruses will be most predominant during the upcoming season."

During this year's flu season in Australia, the vaccine was only 10 percent effective.

States shown in red have reported high levels of flu activity. (CDC)

Still, the CDC recommends that all people over the age of five get the shot to reduce the symptoms of the virus.

"It's not too late to get a flu vaccine — as long as flu is spreading vaccination should continue," the CDC's Kristen Nordlund told weather.com. "It’s important to know that it takes about two weeks for protection to set in."

California has been particularly hard hit, with at least 27 deaths of people under 65 attributed to the flu, the Associated Press reports. As the number of cases continues to climb there, hospitals are beginning to run out of Tamiflu, the anti-viral medication used to treat the illness.

Susan Klein-Rothschild, deputy director of the Santa Barbara County Public Health Department, told weather.com the number of cases of flu reported in the county is significant.

"Our hospitals and health care providers are seeing large numbers of people presenting with influenza-like illness," Klein-Rothschild said. "The trend for this flu season is much greater than our typical flu season."

There have been five outbreaks in Santa Barbara County residential health care facilities and eight deaths attributed to the virus, all of them occurring in individuals over 65.

Flu cases across the United States as of Dec. 30, 2017.(Center for Disease Control and Prevention)

"This strain appears to be particularly strong," Klein-Rothschild said. "A number of people who live in the same facility have tested positive for the flu. It appears to spread easily. We are strongly recommending residents to get vaccinated, use good hand hygiene and stay home if they are ill."

According to the CDC, people at high risk for flu complications include children younger than five, adults 65 years and older and pregnant women. Those with medical conditions such as asthma, chronic lung or heart disease, diabetes and obesity are also at risk for flu complications, including pneumonia.

Anti-virals like Tamiflu are effective in lessening the symptoms of H3N2 but are most effective if administered within 48 hours of the onset of symptoms.

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